fs/proc/array.c: slightly improve render_sigset_t

fs/proc/array.c: slightly improve render_sigset_t

format_decode and vsnprintf occasionally show up in perf top, so I went
looking for places that might not need the full printf power.  With the
help of kprobes, I gathered some statistics on which format strings we
mostly pass to vsnprintf.  On a trivial desktop workload, I hit "%x" 25%
of the time, so something apparently reads /proc/pid/status (which does
5*16 printf("%x") calls) a lot.

With this patch, reading /proc/pid/status is 30% faster according to
this microbenchmark:

	char buf[4096];
	int i, fd;
	for (i = 0; i < 10000; ++i) {
		fd = open("/proc/self/status", O_RDONLY);
		read(fd, buf, sizeof(buf));
		close(fd);
	}

Link: http://lkml.kernel.org/r/1474410485-1305-1-git-send-email-linux@rasmusvillemoes.dk
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Andrei Vagin <avagin@openvz.org>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>